The term "immersion oil" is a familiar technical term in the field of microscopy and related fields and does not signify oils in the ordinary sense, but embraces general immersion media which are suitable for oil immersion optics. In the field of microscopy, for example, oil immersion optics are used at the objective lens and condenser lens of a microscope. The immersion oil used in oil immersion optics forms a part of the optical system and therefore must be precisely adapted to the optical system.
In oil immersion optics the immersion oil completely fills the space between objective and cover glass or preparation to be investigated. In oil immersion microscopic investigation there is obtained a higher resolution and light intensity as well as a substantially less spherical aberration than in a dry optical system (refractive index n.sub.D of air 1.000) or in a water immersion optical system (refractive index n.sub.D of water 1.332).
In oil immersion microscopy the refractive index n.sub.D of the immersion oil should be adapted at least to the glass of the objective front lens, i.e. should amount to about 1.500 to about 1.525 (at 23.degree. C.). According to agreement of microscope manufacturers and corresponding regulations of the German Institute for Standards, an immersion oil should have the following values: n.sub.D (23.degree. C.)=1.515, n.sub.e (23.degree. C.)=1.518.+-.0.0004 and dispersion .nu..sub.e =44.+-.5.
Further, the immersion oil should have good UV-transmissivity, should be as fluorescence-free as possible and should neither affect glass, synthetic object carriers nor most of the samples to be investigated. Moreover, the immersion oil should be colorless, should be as odorless as possible, should not be hygroscopic, should have a viscosity which allows it to be easily handled, should have no harmful effect on the user and should exhibit a constant composition, i.e. no optical change should occur by the influence of light, air, temperature and the like.
The hitherto known immersion oils consist for the most part of mixtures of mineral oils, paraffin oils and/or polyisobutylenes with compounds having a high refractive index such as, for example, polychlorinated biphenyls (PCB) or hydrogenated terphenyls as disclosed in U.S. Pat. No. 3,929,667.
The PCB compounds are known to be toxic and environmentally dangerous and are therefore used only when it is absolutely necessary because of their favorable fluorescence properties. The other compounds such as, for example, the hydrogenated terphenyls have on the other hand the disadvantage of a moderate to slight, but still troublesome fluorescence in UV-light and a worse transmissivity. Moreover, PCB compounds affect synthetic materials such as, for example, synthetic object carriers.
Water-soluble immersion oils have the advantage that the cleaning of the objective and objective carrier is substantially facilitated. On the other hand, in the case of water-soluble immersion oils there exists the danger that coloring substance is dissolved out from noncovered, colored microscopic sections or smear preparations (e.g. blood smears) or insufficiently fixed objects are washed away. In such cases there is therefore advantageously used a non water-soluble immersion oil.